Neurosteroids are synthesized in the central and peripheral nervous system, particularly in myelinating glial cells, but also in astrocytes and many neurons and act in the nervous system (Paul, S. M., Purdy, R. H. Neuroactive steroids. FASEB J. 1992, 6, 2311–22; Robel, R., Baulieu, E. E. Neurosteroids biosynthesis and function. Trends Endocrinol. Met 1994, 5, 1–8; Kulkarni, S. K., Reddy, D. S. Neurosteroids: A new class of neuromodulators. Drugs Today 1995, 31, 433–55; Compagnone N. A., Mellon S. H. Neurosteroids: Biosynthesis and Function of These Novel Neuromodulators. Frontiers in Neuroendocrinology 2000, 21, 1–56). Research over the past decade has elucidated their multiple effects on various neurotransmitter systems (Majewska, M. D. Neurosteroids: Endogenous bimodal modulators of the GABAA receptor. Mechanism of action and physiological significance. Prog. Neurobiol. 1992, 38,379–95; Lambert, J. J, Belelli, D., Hill-Venning, C., Peters, J. A. Neurosteroids and the GABAA receptor function. Trends Pharmacol. Sci. 1995, 16, 95–303; Schumacher, M., Robel, P., Baulieu, E. E. Development and regeneration of the nervous system: A role for neurosteroids. Dev. Neurosci. 1996, 18, 16–21). The neuroactive steroids, allopregnanolone, allotetrahydrodeoxycorticosterone and progesterone act as allosteric agonists of the GABAA receptor and potentiate GABAA receptor Cl− channel responses (Majewska, M. D. Neurosteroids: Endogenous bimodal modulators of the GABAA receptor. Mechanism of action and physiological significance. Prog. Neurobiol. 1992, 38,379–95; Lan, N. C., Gee, K. W. Neuroactive steroid actions at the GABAA receptor. Horm. Behav. 1994, 28, 537–44; Goodnough, D. B., Hawkinson, J. E., Neuroactive steroid modulation of [3H]muscimol binding to the GABAA receptor complex in rat cortex. Eur. J. Pharmacol. 1995, 288, 15762; Gee, K. W., McCauley, L. D., Lan, N. C., A putative receptor for neurosteroids on the GABAA receptor complex: The pharmacological properties and therapeutic potential of epalons. Ctit. Rev. Neurobiol. 1996, 9, 207–27). Pregnenolone sulfate and dehydroepiandrosterone sulfate however, have been shown to decrease GABA-mediated chloride currents (Mienville, J. M., Vicini, S., Pregnenolone sulfate antagonizes GABAA receptor-mediated currents via a reduction of chanel opening frequency. Brain Res. 1989, 489, 1904).
The GABAA receptor complex can exist as multiple isoforms and demonstrate a variety of pharmacological profiles that arise from their multimeric structure and the diversity of their component membranesubunits. The function of bezodiazepines requires a GABAA receptor complex comprised of α, β, and γ subunits, while t-butylbicycloorthobenzoate (TBPS, a noncompetitive antagonist at the receptor complex ), barbiturates, and neuroactive steroids will modulate the receptor complex comprised of only α and β subunits.
Recently, the existence of an additional novel modulatory site on gamma-amino butyric acid GABAA receptor complex for specific steroid metabolites, such as 5a-pregnan-3α-ol-20-one (5PG), a reduced progesterone metabolite, was demonstrated pharmacologically in brain homogenates and in expressed recombinant receptors. This steroid binding site is functionally coupled to other modulatory sites on the GABAA receptor complex (Lan, N. C., Chen, J-S., Johnson, D., Gee K. W. Differential effects of 4′-chlordiazepam on expressed human GABAA receptors. J. of Neurochem. 1995, 684–88). Electrophysiological studies performed on expressed GABAA receptors have shown modulatory and direct effects of steroids. According to Gee et al. (Gee K. W., Bolger, M. B., Wieland, S., Belleli, D., and Chen, J. S. Pharmacology of a GABAA receptor coupled steroid recognition site. Synaptic Transmission 1992, 111–17) and Wilson (Wilson, M. A., Influences of gender, gonadectomy, and estrous cycle on GABA/BZ receptors and benzodiazepine responses in rats. Brain Res. Bull 1992, 165–72) benzodiazepines and steroid hormone derivatives can potentiate the inhibitory actions of GABA through interactions with the GABAA/BZ/chloride channel complex Binding of these steroid analogs to their respective site on the GABAA/BZ/chloride channel complex causes a modification of all other receptor sites within the complex, including the benzodiazepine site. Therefore, neurosteroids allosterically enhance the binding of a benzodiazepine to the enzodiazepine receptor site.
The 3α-hydroxylated pregnane steroids have been shown to be potent anticonvulsants (Kokate T. G., Svensson, B. E., Rogawski M. A. Anticonvulsant activity of neurosteroids: Correlation with gamma-aminobutyric acid-evoked chloride current potentiation. J. Pharmacol. Exp. Ther. 1994, 279, 1223–1229; Frye C. A., The neurosteroid 3a,5a-THP has antiseizure and possible neuroprotective effects in an animal model of epilepsy. Brain Res. 1995, 696, 113–120), anxiolytics (Crawley J. N., Glowa J. R., Majewska M. D., Paul S. M. Anxiolytic activity of an endogenous adrenal steroid. Brain Res. 1986, 398, 382–5; Birtran D., Hilvers R. J., Kellogg C. K., Anxiolytic effects of 3α-hydroxy-5α[β]-pregnan-20-one: Endogenous metabolites of progesterone that are active at the GABA-A receptor. Brain Res. 1991, 561, 157–161; Birtran D., Shiekh M., McLeod M. J. Neuroendocninol. 1995, 7, 171–177) and antistress agents (Purdy R. H., Morrow A. L., Moore Jr P. H., Paul S. M. Stress-induced elevations of gamma-aminobutyric acid type-A active steroids in rat brain. Proc. Natl. Acad. Sci. USA 1991, 88, 45534557; Barbacia M. L., Roscetti G., Bolacchi F., Concas A., Mostallino M. C., Purdy, R. H., Biggio G. Stress-induced increase in brain neuroactive steroids: Antagonism by abecarnil. Pharmacol. Biochem. Behav. 1996, 54, 205–210) while, sulfated neurosteroids enhance memory performance in rodents (Floods J. F., Smith G. E., Roberts E. Dehydroepiandrosterone and its sulfate enhance memory retention in mice. Brain Res. 1988, 447, 269–278; Mayo W., Dellu F., Robel P., Cherkaoui J., Le Moal M., Baulieu E. E. Simon H. Infusion of neurosteroids into the nucleus basalis magnocellularis affects cognitive processes in the rat. Brain Res. 1993, 607, 324–328). The GABAA agonistic neurosteroids have been the subject of many publications (Han, M.; Hu, Y.; Zorumski, C. F.; Covey, D. F.; J. Med. Chem, 1995, 38, 4548–4556; Hu, Y.; Zorumski, C. F.; Covey, D. F.; J. Med. Chem., 1993, 36, 3956–3967; Anderson, A.; Boyd, A. C.; Byford, A.; Campbell, A. C.; Gemmell, D. K; Hamilton, N. M.; Hill, D. R.; Hill-Venning, C.; Lambert, J. J.; Maidment, M. S.; May, V.; Marshall, R. J.; Peters, J. A.; Rees, D. C.; Stevenson, D.; Sundaram, H.; J. Med. Chem., 1997, 40, 1668–1681; Hogenkamp, D. J.; Tahir, S. H.; Hawkinson, J. E.; Upasani, R. B.; Alauddin, M.; Kimbrough, C. L.; Acosta-Burruel, M.; Whittemore, E. R.; Woodward, R. M.; Lan, N. C.; Gee, K. W.; Bolger, M. B.; J. Med. Chem., 1997, 40, 61–72; Upasani, R. B.; Yang, K. C.; Acosta-Burruel, M.; Konkoy, C. S.; McLellan, J. A.; Woodward, R. M.; Lan, N. C.; Carter, R. B.; Hawkinson, J. E.; J. Med. Chem., 1997, 40, 73–84) and of several patents (U.S. Pat. No. 6,143,736, U.S. Pat. No. 5,939,545, U.S. Pat. No. 5,925,630, EP01038880, U.S. Pat. No. 5,591,733, WO 96116076, WO 95/21617, WO 94/27608, U.S. Pat. No. 5,232,917, WO 93/18053, WO 93/05786, WO 93/03732, US RE 035517, WO 91116897).